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Title: Surface modification of AISI 316L stainless steel using different forms of NiTi for improving cavitation erosion resistance
Authors: Chiu, Ka-yu
Degree: M.Phil.
Issue Date: 2005
Abstract: AISI 316L stainless steel is widely used in hydraulic machinery and in liquid-handling systems mainly because of its excellent corrosion properties, good machinability, and relatively low cost. For such applications, cavitation erosion may become a major mode of attack under certain circumstances. Due to its low hardness (=200 HV), the cavitation erosion resistance of 316L may not be adequate in harsh environments. In view of this low erosion resistance, the range of applications of 316L stainless steel may thus be limited, albeit it has desirable properties in other respects. It is the aim of the present project to improve the cavitation erosion resistance of 316L via surface modification. It is well known that nickel titanium (NiTi) possesses superior cavitation erosion resistance mainly because of its superelasticity. In addition NiTi possesses a high corrosion resistance comparable to that of stainless steel. These desirable mechanical and electrochemical properties of NiTi make it a good candidate as a cladding material for 316L stainless steel. In the present study, three surface modification methods have been employed, using different forms of NiTi. These include: Method (1): laser surface alloying (LSA) of 316L using NiTi powder, Method (2): laser cladding (LC) on 316L using NiTi strips, and Method (3): cladding of 316L with NiTi plate by microwave-assisted brazing (MWAB). These methods, each with its own limitations and merits, are capable of significantly increasing the cavitation erosion resistance Re, the improvement reached being in the range of 20-40 folds. The increase in Re is lowest in Method (1) and highest in Method (3) because of the difference in the degree of dilution of NiTi by the substrate. A low dilution of NiTi in Method (3) allows the cladding to retain a large portion of the superelasticity of bulk NiTi. On the other hand, the bonding strength achieved in Method (3) is relatively low because bonding was formed via diffusion rather than fusion as in the other two methods. Since surface modification against cavitation erosion is usually applied at locations of an engineering part that are prone to attack, galvanic effect between the cladding and the neighboring substrate is expected to be present. Nevertheless, the galvanic effect between the NiTi cladding and the 316L substrate is only minimal as evidenced by a low galvanic current density in the NiTi-316L couple for all the three cladding methods. Hydrogen embrittlement of Ti-based alloys is another concern for applications in aqueous environments. In view of this, the effect of hydrogen uptake on the cavitation erosion of the cladding was investigated using electrolytic hydrogen charging. The cavitation erosion resistance is reduced due to the formation of hydrides, or to hydrogen induced softening. The effect of hydrogen might not be a real threat since the NiTi-modified part is anodic to the neighboring steel and is protected from hydrogenation. The present study has demonstrated the feasibility of improving the cavitation erosion resistance of AISI 316L using NiTi in different forms.
Subjects: Hong Kong Polytechnic University -- Dissertations.
Steel, Stainless.
Lasers -- Industrial applications.
Surface mount technology.
Pages: xv, 158 leaves : ill. ; 30 cm.
Appears in Collections:Thesis

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